<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>闭包陷阱</title>
</head>
<body>
  <button id="btn">add</button>
    
  <script>
    function foo() {
      var el = document.getElementById('btn');

      el.onclick = function() {
        console.log(el.id);
      }
    }

    foo();
  </script>
</body>
</html>